1. Field of the Invention
This invention relates to the inhibition of scale formation from oil well brines utilizing a slow release inhibitor composition.
2. Information Disclosure Statement Including Description of Related Art
The following information is disclosed in accordance with the terms of 37 CFR 1.56, 1.97 and 1.98.
In the production of crude oil from underground wells, mineral scales such as barium sulfate, strontium sulfate, calcium sulfate and calcium carbonate are often formed on wellbore and equipment surfaces where oil and water containing dissolved salts are coproduced. The formation of scale can slow oil production rate and, at the extreme, stop production completely. An expedient often utilized is to inject or "squeeze" a solution of a scale inhibitor such as a polyphosphonate or a polyacrylic acid into the reservoir rock and allow the absorbed inhibitor to desorb during fluids production. In practice, however, the desorption process is often found to be quite rapid once production is resumed necessitating frequent shutdowns for additional treatments. This has the effect of substantially reducing the productivity of the well.
Various improvements in the adsorption and desorption of scale inhibitors have been proposed, some of which are disclosed in the following prior art references.
U.S. Pat. No. 3,654,993, issued Apr. 11, 1972, to Smith et al., teaches the injection into a geological subterranean formulation of an aqueous, acidic solution of a corrosion inhibiting composition comprising any of various complex corrosion inhibitors including relatively high molecular weight "ureas" and "amines," and a polyacrylic acid or salt thereof. Subsequent to the injection of the solution into the formation, a base is injected causing a precipitate to form in the formation which is sparingly soluble in the liquids produced in the formation thus preventing the inhibitor from being rapidly washed out by the produced liquids.
U.S. Pat. No. 3,827,977, issued Aug. 6, 1974 to Miles et al. discloses the in situ deposition in the porous rock formation adjacent to a well bore of a polyvalent metal salt of a polyacrylic acid or partially hydrolyzed polyacrylamide inhibitor by introducing into the porous formation a strongly acidic aqueous solution of a salt of the inhibitor and the polyvalent metal. The acid is partially neutralized by the reservoir liquid causing the polyvalent salt of the inhibitor to phase separate on the porous surfaces.
U.S. Pat. No. 4,357,248 issued Nov. 2, 1982 to Berkshire et al., teaches the inhibition of scale formation in a producing well by injecting a self-reacting alkaline aqueous solution of a pH-lowering reactant, a scale inhibitor and a compound containing multivalent cations, which solution subsequently precipitates a slightly soluble scale inhibitor within the reservoir.
U.S. Pat. No. 4,602,683, issued Jul. 29, 1986 to Meyers, discloses the inhibition of scale deposition during the operation of an oil well by injecting an inhibitor solution into a subsurface brine producing formation at a higher first pH, and thereafter subjecting the solution to a fluid or substance which reduces the higher first pH to a lower second pH causing precipitation of the scale inhibitor in the formation.
None of the foregoing references discloses or suggests the injection into a well reservoir of an aqueous acid solution containing a scale inhibitor, multivalent metal ions, and a heat-sensitive substance which decomposes at the temperature of the reservoir liberating an alkaline compound, thus raising the pH of the solution and causing a multivalent metal salt of the scale inhibitor to phase separate out onto the surfaces of reservoir rock formation.
Parent application Ser. No. 07/837,544, filed Feb. 14, 1992, now U.S. Pat. No. 5,141,655 discloses and claims a method for inhibiting scale formation in oil well reservoir surfaces by injecting an aqueous solution containing a scale inhibitor, multivalent metal ions, and a heat-sensitive, pH-increasing substance which may be urea or any of certain urea derivatives, an acyl azide, or cyanic acid.